Measurement of heterogeneous media in a continuous fluid phase, particularly solids in a fluid, is of commercial interest. Drilling fluids, slip casting, waste disposal, catalytic processes, paper manufacture, and many other commercial processes rely upon particles or solids in fluids. The development of practical and reliable methods for inspecting such fluid systems is of great interest because the properties of the fluid systems impact the quality of the resulting products and/or the energy efficiency of the process.
While there are several kinds of commercial equipment for particle size measurements based on the ultrasonic attenuation methods, these methods are limited by the mathematical inversion step for general cases in which the nature of the particle size distribution and physical properties of the particles are not known a priori. Furthermore, these methodologies do not completely account for the complexities of settling slurries.
The present invention is based, in part, on the fact that during settling a series of interfaces may develop and the fluid stratifies as a function of both particle size and shape. For instance, non-spherical particles orient preferentially during settling. The settling rate can depend upon the local solids loading, particle size distribution and solution (fluid medium) chemistry. In moving from the top to the bottom of a settling column, the particle sizes, shapes, composition, and concentrations can all be continually varying. The dynamics of suspension settling, controlled in part by the fluid medium properties, becomes a means to discriminate the particulate constituents in a suspension or slurry. By positioning ultrasonic interrogations at fixed locations along the settling column and monitoring the ultrasonic responses as a function of time provides a uniquely powerful means to characterize slurries and suspensions.
Moreover, while suspension characterization is important in a variety of situations, the accurate and reliable determination of slurry and suspension properties becomes increasingly important for systems where the properties are changing over time. For example, in paper manufacture, wood pulp is typically refined in a continuous operation where a slurry of wood pulp is processed to change the fiber characteristics, typically by passing the wood pulp through a pair of closely spaced rollers or rotating plates. The purpose of the refining is to change the characteristics of the fiber, which changes can involve splitting the outer and inner layers of the fibers, the splintering of smaller pieces of fiber or other debris and/or shortening of the length of some of the fibers. It is generally desirable to refine the pulp under conditions such that the fibers split without excessively shortening them or making significant amounts of debris. Accurate and reliable determination of pulp properties, such as the degree of refining and consistency of the pulp, can provide useful information for controlling this refining process or adjusting other aspects of the process to make a quality product from the wood pulp.
One common technique for determining the degree of refining of a wood pulp suspension is the freeness test, which is based on measuring the rate at which water drains from a pulp suspension through a wire mesh on which the fibers are retained in the form of a loose mat. Generally, the higher the degree of refining, the slower the water drains. One standard measure of freeness is termed the Canadian Standard Freeness (CSF).
Unfortunately, as well as being time consuming and labor intensive, the freeness test is an imperfect test that is not an accurate measure of the more important pulp qualities. For example, the freeness test is more strongly influenced by the presence and concentration of fines in the pulp than by the physical condition of the fibers. In addition, the refining process causes both fibrillation of the fibers and the rupture of the fibers' internal bonds such that the fibers become soft and swollen. The second effect, swollen fibers, results in improvement in the tensile strength properties of the paper. However, the freeness test is more a measure of fibrillation than of fiber swelling. Accordingly, it is desirable to characterize the degree of refining of wood pulp with a measure other than the freeness. See for example U.S. Pat. No. 4,159,639 to Simms et al. which is hereby incorporated by reference to the extent not inconsistent with the present disclosure.
Unfortunately, the ability to rapidly, reliably, and efficiently monitor suspension properties is limited. Accordingly, it is an object of the present invention to provide improved systems and techniques for determining properties of suspensions. While the present invention is broadly applicable to determining suspension properties for a variety of industrial suspensions in a variety of applications, in one form it is an object of the present invention to provide an apparatus and method to determine the properties of pulp fibers in a pulp suspension, as modified by the refining process, to facilitate prediction of the properties of the to-be-manufactured paper. In another form it is the object of the present invention to determine the particle properties of a settling solid-liquid suspension